1. Field of the Invention
The present invention relates to a controller for controlling a step number of a step motor or a rotational position of a rotor of the step motor, and more specifically to such a controller for a step motor which can satisfactorily follow up a target step number which may be varied with time. For example, when a step motor is employed to actuate a throttle valve in accordance with a depressed amount of an accelerator pedal of a vehicle, the target step number of the step motor will be varied with time, due to change in the depressed amount of the accelerator pedal.
2. Description of the Prior Art
An example of the prior art for controlling a step motor so as to have an actual step number conforming with a target step number which may be varied with time is described in Japanese Laid-Open Patent Publication No. 61-138855. In this prior art, a step number deviation is derived from the target step number and an actual step number so as to adjust the duration of driving pulses in accordance therewith. Specifically, as the deviation is greater, the step motor is driven at a higher speed by reducing the pulse duration, while as the deviation becomes smaller, the pulse duration is increased to drive the step motor at a lower speed, until the deviation reaches zero, when the step motor is stopped. Then the actual step number will conform with the target step number. When this process is repeated each time the target step number varies, the actual step motor will follow up the target step number which varies with time.
Once the speed of the step motor becomes zero, a rotor of the step motor causes vibration. If the step motor is inadvertently restarted thereafter, the vibration will be amplified, which may result in step out. The term "step out" is used herein to refer to a condition in which the rotational position of a rotor does not correspond to the position of coils to which current is applied. Therefore, there has been proposed a technique that, when the actual step number of the step motor has reached the target step number, the pulse for maintaining the actual step number at the target step number is kept on, until the vibration of the rotar is sufficiently damped. In other words, output of a pulse for advancing the step motor to a next step number for restart thereof is inhibited for a certain duration. For example, in Japanese Laid-Open Patent Publication No. 1-185195 , restart is inhibited for a duration of 100 ms, a hundred times longer than the pulse duration (pulse width) corresponding to the highest speed of the step motor.
The above prior art is sufficient in the case where the target step number is gradually varied with time. The prior art will be, however, accompanied with following problems, in the case where the target step number is rapidly varied with time.
FIG. 3 is a diagram illustrating the relation between time and the step number, with time shown by the axis of abscissa and the step number by the axis of ordinate, and the target step number is shown by a stepped linear graph TS. When it is assumed that energization of the step motor to advance to a next step number is started at a timing T0, a step deviation DS (T0) between a target step number at the timing T0 and an actual step number at the timing T0 is derived to determine a duration .DELTA.T0 of a pulse for advancing the step motor to the next step number. When the duration .DELTA.T0 has elapsed since the timing T0, or at a timing T1, a step deviation DS(T1) is again derived to determine a pulse width .DELTA.T1 in accordance therewith. As the step deviation is greater, a shorter pulse width is determined to drive the motor at a higher speed, while as the deviation becomes smaller, a longer pulse width is determined to drive the motor at a lower speed. At a timing T2 when one more pulse permits the actual step number of the motor to conform with the target step number, the .pulse for maintaining the motor at the next step (the target step number) is kept on for a duration (.DELTA.T2 in FIG. 3) long enough to sufficiently damp vibration of a rotor of the motor.
In the case of a motor whose target step number is rapidly varied with time, the target step number may have changed before a timing T3 in FIG. 3 when restart of the motor is permitted. In such a case, a step deviation DS (T3) is derived at the timing T3 when restart is permitted to determine the pulse width .DELTA.T3 again. Therefore, the step deviation DS(T3) has rather increased, and the step motor is required to repeat such an operation as to abruptly remove the increased deviation, as shown by dotted lines in FIG. 3. The alphabetic symbol SP in FIG. 3 illustrates the change of the step number when the step motor which has been at rest is accelerated to the highest speed.
The above description relates to the case in which, in the duration of the pulse started at the timing T2 for conforming the actual step number with the target step number and for damping the vibration of the rotar at the target step number, the target step number has been varied. The same problem will occur in case the actual step number has not reached the target step number. It is assumed in FIG. 4 that a duration of energization .DELTA.T4 is determined in accordance with a deviation DS(T4) at a timing T4. It is also assumed that thereafter the target step number has been varied in the duration of energization .DELTA.T4. In such a case, the actual step number is controlled as shown by dotted lines in FIG. 4. The step deviation at the timing T4 should be however, the deviation in relation to the varied target step number DS(T4)A to determine the pulse width .DELTA.T4A in accordance therewith. This will permit the actual step number to better follow up of variation in the target step number, as shown in solid lines in FIG. 4.
As described above, the prior art determines the pulse width or duration of energization in accordance with the step deviation at start of energization of each pulse, and is consequently ineffective to follow up variation in the target step number thereafter. It has been desired to develop a technique which permits better follow up of any variation in the target step number which may occur after start of pulse energization.